Magnetic recording (“MR”) media and devices incorporating such media are widely employed in various applications, particularly in the computer industry for data/information storage and retrieval applications, typically in disk form. Conventional magnetic thin-film media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording medium layer, are generally classified as “longitudinal” or “perpendicular”, depending upon the orientation of the magnetizations of the grains of magnetic material.
A conventional longitudinal recording, hard disk-type magnetic recording medium 1 commonly employed in computer-related applications is schematically illustrated in FIG. 1, and comprises a substantially rigid, non-magnetic metal substrate 10, typically of aluminum (Al) or an aluminum-based alloy, such as an aluminum-magnesium (Al—Mg) alloy, having sequentially deposited or otherwise formed on a surface 10A thereof a plating layer 11, such as of amorphous nickel-phosphorus (Ni—P); a bi-layer 12 comprised of a seed layer 12A of an amorphous or fine-grained material, e.g., a nickel-aluminum (Ni—Al) or chromium-titanium (Cr—Ti) alloy, and a polycrystalline underlayer 12B, typically of Cr or a Cr-based alloy; a magnetic recording layer 13, e.g., of a cobalt (Co)-based alloy with one or more of platinum (Pt), Cr, boron (B), etc.; a protective overcoat layer 14, typically containing carbon (C), e.g., diamond-like carbon (“DLC”); and a lubricant topcoat layer 15, e.g., of a perfluoropolyether. Each of layers 10–14 may be deposited by suitable physical vapor deposition (“PVD”) techniques, such as sputtering, and layer 15 is typically deposited by dipping or spraying.
In operation of medium 1, the magnetic layer 13 is locally magnetized by a write transducer, or write “head”, to record and thereby store data/information therein. The write transducer or head creates a highly concentrated magnetic field which alternates the media magnetization direction based on the bits of information to be stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the material of the recording medium layer 13, the grains of the polycrystalline material at that location are magnetized. The grains retain their magnetization after the magnetic field applied thereto by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The magnetization of the recording medium layer 13 can subsequently produce an electrical response in a read transducer, or read “head”, allowing the stored information to be read.
Efforts are continually being made with the aim of increasing the recording (areal) density, i.e., the bit density, or bits/unit area, and signal-to-medium noise ratio (“SMNR”) of the magnetic media. In this regard, so-called “perpendicular” recording media have been found to be superior to the more conventional “longitudinal” media in achieving very high bit densities. In perpendicular magnetic recording media, residual magnetization is formed in a direction perpendicular to the surface of the magnetic medium, typically a layer of a magnetic material on a suitable substrate. Very high linear recording densities are obtainable by utilizing a “single-pole” magnetic transducer or “head” with such perpendicular magnetic media.
Efficient, high bit density recording utilizing a perpendicular magnetic medium requires interposition of a relatively thick (as compared with the magnetic recording layer), magnetically “soft” underlayer (“SUL”) layer, i.e., a magnetic layer having a relatively low coercivity of about 2–150 Oe, such as of a NiFe alloy (Permalloy), between the non-magnetic substrate, e.g., of glass, aluminum (Al) or an Al-based alloy, and the magnetically “hard” recording layer having relatively high coercivity of several kOe, typically about 4–10 kOe, e.g., of a cobalt-based alloy (e.g., a Co—Cr alloy such as CoCrPtB) having pendicular anisotropy. The magnetically soft underlayer serves to guide magnetic flux emanating from the head through the hard, perpendicular magnetic recording layer.
A typical perpendicular recording system 20 utilizing a magnetic medium 1′ with a relatively thick soft magnetic underlayer, a relatively thin hard perpendicular magnetic recording layer, and a single-pole head, is illustrated in FIG. 2, wherein reference numerals 10, 11, 3, 4, and 5, respectively, indicate a non-magnetic substrate having a surface 10A, an adhesion layer (optional), a soft magnetic underlayer, at least one non-magnetic interlayer, and at least one perpendicular hard magnetic recording layer. Reference numerals 7 and 8, respectively, indicate the single and auxiliary poles of a single-pole magnetic transducer head 6. The relatively thin interlayer 4 (also referred to as an “intermediate” layer), comprised of one or more layers of non-magnetic materials, serves to (1) prevent magnetic interaction between the soft underlayer 3 and the at least one magnetically hard recording layer 5 and (2) promote, desired microstructural and magnetic properties of the at least one hard recording layer.
As shown by the arrows in the figure indicating the path of the magnetic flux φ, flux φ is seen as emanating from single pole 7 of single-pole magnetic transducer head 6, entering and passing through the at least one vertically oriented, hard magnetic recording layer 5 in the region below single pole 7, entering and traveling within soft magnetic underlayer 3 for a distance, and then exiting from the soft magnetic underlayer and passing through the at least one perpendicular hard magnetic recording layer 5 in the region below auxiliary pole 8 of single-pole magnetic transducer head 6. The direction of movement of perpendicular magnetic medium 1′ past transducer head 6 is indicated in the figure by the arrow above medium 1′.
With continued reference to FIG. 2, vertical lines 9 indicate grain boundaries of polycrystalline layers 4 and 5 of the layer stack constituting medium 1′. Since magnetically hard main recording layer 5 is epitaxially formed on interlayer 4, the grains of each polycrystalline layer are of substantially the same width (as measured in a horizontal direction) and in vertical registry (i.e., vertically “correlated” or aligned).
Completing the layer stack is a protective overcoat layer 14, such as of a diamond-like carbon (DLC), formed over hard magnetic layer 5, and a lubricant topcoat layer 15, such as of a perfluoropolyether material, formed over the protective overcoat layer.
As indicated supra, efforts are continually being made with the aim of increasing the recording (areal) density, i.e., the bit density, or bits/unit area, and signal-to-medium noise ratio (“SMNR”) of the magnetic media. For example, the SMNR may be increased by reducing the grain size of the recording media, as by utilization of appropriately selected seed and underlayer structures and materials, by reduction of the thickness of the magnetic recording layer, and by a novel reactive deposition (oxidation) process. However, severe difficulties are encountered when the bit density of longitudinal media is increased above about 100–180 Gb/in2 in order to form ultra-high recording density media, such as thermal instability, when the necessary reduction in grain size exceeds the superparamagnetic limit. Such thermal instability can, inter alia, reduce and cause undesirable decay of the output signal of hard disk drives, and in extreme instances, result in total data loss and collapse of the magnetic bits.
One proposed solution to the problem of thermal instability arising from the very small grain sizes associated with ultra-high recording density magnetic recording media, including that presented by the superparamagnetic limit, is to increase the crystalline anisotropy, thus the coercivity of the M-H loop of the media, in order to compensate for the smaller grain sizes. However, this approach is limited by the field provided by the recording head.
Another proposed solution to the problem of thermal instability of very fine-grained magnetic recording media is to provide stabilization via ferromagnetic or anti-ferromagnetic coupling of the recording layer with another ferromagnetic layer. In this regard, it has been recently proposed (E. N. Abarra et al., IEEE Conference on Magnetics, Toronto, April 2000) to provide a stabilized magnetic recording medium comprised of at least a pair of ferromagnetic layers (e.g., CoCrPtB layers) which are anti-ferromagnetically-coupled (“AFC”) by means of an interposed thin, non-magnetic spacer layer. The coupling is presumed to increase the effective volume of each of the magnetic grains, thereby increasing their stability; the coupling strength between the ferromagnetic layer pairs being a key parameter in determining the increase in stability.
Referring to FIGS. 3(A)–3(D), illustrated therein, in simplified, schematic perspective view, are several possible magnetization configurations of conventional AFC media, wherein: FIG. 3(A) illustrates a longitudinal medium, wherein the direction of the magnetic moments of each of the grains of both of the anti-ferromagnetically coupled layers is aligned parallel to the substrate surface; FIG. 3(B) illustrates a longitudinal “tilted” medium, wherein the alignment direction of the magnetic moments of each of the grains of one of the anti-ferromagnetically coupled layers (illustratively, the upper layer) is tilted at an angle with respect to the substrate; FIG. 3(C) illustrates a perpendicular “tilted” medium, wherein the alignment direction of the magnetic moments of each of the grains of each of the anti-ferromagnetically coupled layers is tilted at an angle with respect to the substrate; and FIG. 3(D) illustrates a perpendicular medium, wherein the direction of the magnetic moments of each of the grains of both of the anti-ferromagnetically coupled layers is aligned normal to the substrate surface.
A significant drawback is associated with each of the illustrated types or configurations of conventional AFC media when each of the AFC-coupled ferromagnetic layers is granular, i.e., the in-plane grains are discontinuous in nature. As a result, the enhancement of thermal stability is somewhat moderated because the net enhancement is determined merely by the total volume of the grains which are coupled. Specifically, if the magnetic grains of the upper and lower magnetic layers are not grown in vertical alignment, or if they are not of equal size, the areas written in each of the pair of ferromagnetic layers may not coincide. In addition, the prior art approaches to media design fail to adequately take into account the significant effect on stability of magnetic recording media arising from interactions between magnetic grains.
Still another approach for achieving magnetic recording media exhibiting higher areal recording densities along with enhanced magnetic performance characteristics involves formation of media (so-called “CG” media) wherein a pair of vertically stacked magnetic layers, i.e., a so-called “granular” recording layer (wherein the magnetic grains are only weakly exchange coupled laterally) and a continuous layer (wherein the magnetic grains are strongly exchange coupled laterally) are ferromagnetically coupled together. In such CG media, while the entire continuous magnetic layer may couple with each grain in the granular magnetic layer, and thus the continuous magnetic layer will always provide additional strong local exchange coupling which does not aid in defining overall sharp magnetic transitions. More specifically, the enhancement of thermal stability afforded by the usual (i.e., ferromagnetically coupled) CG media is actually obtained with a sacrifice in SMNR or an increase in “jitter” noise.
Accordingly, there exists an acute need for improved methodology and structures for providing thermally stable, high areal recording density magnetic recording media, e.g., in the form of hard disks, with increased signal-to-media noise ratios (SMNRs), e.g., longitudinal media, which methodology and media structures can be implemented/fabricated at a manufacturing cost compatible with that of conventional manufacturing technologies for forming high areal recording density magnetic recording media.
The present invention, therefore, addresses and solves problems attendant upon forming high areal recording density magnetic recording media, e.g., in the form of hard disks, which media utilize magnetic or anti-ferromagnetic coupling between spaced-apart pairs of ferromagnetic layers for simultaneously enhancing thermal stability and increasing SMNR, while providing full compatibility with all aspects of conventional automated manufacturing technology. Moreover, manufacture and implementation of the present invention can be obtained at a cost comparable to that of existing technology.